The arrangement of a printhead used in an inkjet printing apparatus which prints information (e.g., a desired character and image) on a sheet-like print medium (e.g., paper or a film) is disclosed in, for example, Japanese Patent Publication Laid-Open No. 2002-374163. According to this publication, a printing element formed from a heating element (heater), a driver for driving the printing element, and a logic circuit for selectively driving the driver in accordance with image data are formed on a silicon substrate.
Color inkjet printers using a thermal inkjet technique of discharging ink by heat energy are achieving higher resolutions year after year. Particularly in the printhead of the inkjet printer, the layout density of orifices (nozzles) for discharging ink increases from 600 orifices to 1,200 orifices per inch.
The amount of discharge ink droplet which forms each pixel tends to decrease year after year in order to reduce graininess in the gradation of a monochrome image and at middle to low-density portions on a color photographic image. Especially in a printhead which discharges color ink, the amount of discharge ink droplet was about 15 pl a few years ago, but has decreased to 5 pl or 2 pl recently.
However, the following problem occurs when a printhead which prints a high-resolution image by small ink droplets is adopted. That is, when a high-resolution color graphic image or photographic image is printed, a high-resolution, high-quality image is printed to meet the user's need if a printhead discharging a small droplet is employed. To the contrary, in image printing of a color graph on a spreadsheet or the like that does not require high resolution, the use of small ink droplets increases the scan count, and it becomes difficult to meet demands for high-speed printing.
In order to solve this problem, countermeasures must be taken to increase the number of orifices laid out on one array and increase a printable area where printing is done by one scan or to increase the orifice layout density and enable printing a high-resolution image by one scan.
For example, Japanese Patent Publication Laid-Open No. 2002-79672 discloses a printhead in which orifice arrays are staggered and laid out on the two sides of a common ink supply port (common liquid chamber). This enables the layout density of orifice arrays to become double, compared to that of a single array. Even if the ink droplet is downsized, the printing speed does not decrease.
A printhead having the highest orifice layout density has 1,200 orifices per inch, and the mainstream of the discharge ink droplet amount is 2 pl. However, in order to meet demands for higher image qualities, the discharge ink droplet amount must be reduced to 1 pl or less. Moreover, in order to maintain the same printing speed, the number (common supply number) of orifices must be increased. However, if the number of orifices of the printhead is simply increased, the printhead becomes large, which is disadvantageous in cost and size.
In order to decrease the discharge ink droplet amount to 1 pl or less and maintain the printing speed for higher image qualities, the orifice layout density must be double or more the current maximum layout density (1,200 orifices per inch). Note that when orifices are staggered and laid out on the two sides of the ink supply port, the layout density on one side suffices to be 1,200 orifices per inch.
If the orifice layout density is further increased (to 2,400 orifices (1,200 orifices on one side) per inch), it becomes difficult to ensure the necessary width of a partition which separates adjacent nozzles, and the necessary width of an ink channel which greatly influences discharge performance. To solve this problem, there is proposed an arrangement in which orifices laid out on each array are staggered by changing the distance from the common liquid chamber.
When orifices on one array are staggered, the distance from the common liquid chamber to the orifice changes. In this case, in order to attain the same orifice discharge performance between orifices having the different distances, it is considered effective to optimize, in accordance with the position, the shape and size of each electrothermal transducer (heater) for generating heat energy. However, when the shape and size of the heater are changed, driving energy applied to the heater must be adjusted in accordance with the shape.
To adjust the driving energy, it is considered to change, in accordance with the heater shape, a signal for determining the time during which electric energy is applied to the heater. In this case, however, the number of signal lines increases. In a general inkjet printhead, this leads to an increase in the number of terminals which connect the printhead to the apparatus main body since signals from the printing apparatus main body are supplied through these signal lines
Also, the printing apparatus main body does not have a sufficient margin for the number of signal lines in a cable for supplying signals to the printhead. Hence, the printing apparatus main body must be modified to increase the number of signals supplied via the cable and increase the number of ASIC ports in order to output the signals.
These changes undesirably increase the size and cost of an inkjet printing apparatus of which reduction of size and cost is strongly demanded.
This problem is not limited to only a thermal inkjet printhead but common to all printheads having at least two types of printing elements which require different application electric energies (powers) in order to obtain a desired printing characteristic.